Means for signal amplifying and mixing utilizing space charge neutralization



Oct. 30, 1951 J COLEMAN 2,573,247

MEANS FOR SIGNAL AMPLIFYING AND MIXING UTILIZING SPACE CHARGE NEUTRALIZATION Filed Jan. 3, 1949 2 SHEETS-SHEET l I 4 I e 5 p a l r am ur d4 NVENTOR Jbkrbfi @kmam ATTORNEY Oct. 30, 1951 2,573,247

J. H. COLEMAN MEANS F SIGNAL A LIFYING AND MIXING UTILIZ SPACE C GE NEUTRALIZATION 2 SHEETS-SHEET 2 Filed Jan. 3, 1949 INVENTO R fiknlfhkmn/ Patented Oct. 30, 1951 MEANS SIGNAL AMPLIFYING AND MIX- ING'UTILIZING SPACE CHARGE NEUTRAL- IZATION John H. Coleman, Princeton, J assignor to Radio Corporation of America, a corporation of Delaware Application J anuary 3, 1949, Serial No. 68,947

11 Claims.

1 iii-ismyention relates generally to high current electronqgun devices and systems and more partnularly to the combination of such devices in apparatus such as oscilla ors, amplifiers, mod- Metersand mixers wherein the density of an electron beam is varied, that is, the beam is conv ontrasted and dispersed by variations of space charge neutralization and with negli a companying electron velocity variations.

In applyin electron guns to microwave devices and their associated circuitait has been most desirable the past that the electron beam be e, denserand concentrated. These qualities have been somewhat improved by the use of various electron Iocussing devices and particulariiorms of field-tree arrangements, none of which are similar, individually or in combination, with the methods and apparatus disclosed h8Iein and set forth in the appended claims. U S. Patent 2,111,940 discloses, for example, a focussed electron beam in a metal envelope that purports to establish a field-free space. The beam is shielded from extraneous fields only the :metal envelope, .as in a wave-guide structure, but it is definitely stated that the ionization potentialrof the gas filling is not reached .(lines 2ll,. .2l,-xcolumn 1,, page 33. The space between electrode .5 and chamber wall 2 in this patent is, thereiore, not field-free in that the beam itself produces afield as itconsists'of charged particles. In H. 8.. :Patent 1,565,873 isdisclosed the neutralization of the space char throughout the whole general space through which passes a neriodof cathode rays. There is nosuggestion in. this patent that aheavy current beam, that is being focussed at a point, is concentrated by the trappingof ions within the beam itselt. ,Fur-

thermore, in the -.devices disclosed herein the gas Pressures are not criticaL However, neither of the. reference patents citedisrthere disclosed the concentration of the beam and the dispersion of the beam as ,a step inthermethod of using the phenomenon of ion trapping for new and. useful purposes of generating; modulatin amplifying and mixing 10f currents. Likewise, the patents citeddo not dis- .in-copending application :filed .on December ?31, mail-sin the name of Ernest-G. Linder, Serial No.

close 'the structures described and claimed here- 68,605, there is disclosed and claimed'zthe method ofamd aippaizatusioricontrollingtherlensity of an electron beam: and the combination of the controlled beam mith'microwave oscillator :and mod- This control of the density of the beam is-accomplished by establishing a field-free region in the medium through which the. beam passes, whereby the ions, formed by the ionization of the residual-atoms in the gas of the medium by collisions with the electrons of the beam, .are trappedwithin the beam and neutralize the space charge therein. Control is thereby had of the sizeiof the beam. It may be reduced .or dispersed at its approaches the apertures in the resonant cayityof an oscillator or the aperture :in the .colleoting structure of a modulator .or amplifier. Means is also provided in said copending system for employing a magnetic field to .assist in .concentrating the beam.

When the improved beam device is combined in.;a structure, such as an oscillator, the lefticiency of the system is increased by the improved beam concentration, thus providing for a larger percentage of the electrons to pass through the restricted aperture in the resonant .cavity of the oscillator, Also, dispersion in the drift s ace ,of the oscillator is reduced, with attending decrease in dehunching. When the improved beam r de-- vice is combined .with .a modulator .or amplifier structure, the efliciency .oI the device is increased becausethe beam is denser and finer and smaller apertures may :be .used. With such apertures .and fine beams, ;a small dispersion results in a large proportion of. the electrons not passing through the aperture, which is .a measure .of the .efilciency of the :device .as .an amplifier or modulator.

-Etlhepresent invention is an improvement over that disclosed in the said application in that there is provided herein a separate electrode to vary or control the space charge neutralization provided by trapping the positive ions in the electron beam and hence the degree of fineness or dispersion of the beam. Were'an accelerating electrode used for this purpose, as disclosed in said application, the currents in the varying or modulating circuit would be larger and there would be less freedom of choice in the selection of the various voltages for different geometries of cons'truetion and arrangements of the involved devices.

In this application the invention is also disclosed in combination with the structures of amplifiers, oscillators and mixers of currents of different frequencies, 'to obtain a current of an intermediate frequency. In the latter embodimentof the invention there is provided the combination in the one device of two methods of beammodulation by the use of two separate electrodes to accomplish mixing, one of the electrodes being a control grid adjacent the cathode, and the other electrode providing control of space charge neutralization within the field-free space.

Among the objects of the invention is to provide a method of and apparatus for producing a high current density electron beam.

Another object of the invention is to provide a method of and apparatus for controlling the density of an electron beam. Another object of the invention is to provide a field-free space in the region of an electron beam.

to neutralize the space charge within the beam. Another object of the invention is to control Faraday cage 8, which is connected to suitable conventional indicating instruments, not shown,

7 to observe the currents of the beam passing the density of an electron beam as it approaches an aperture, to control the proportion of electrons that pass therethrough and thereby accomplish current modulation.

Another object is to provide improved methods I of and means for controlling the space charge neutralization of an electron beam device and system.

. A further object is to provide improved methods of and means for generating, amplifying or mixing or modulating electric currents by controlling spacecharge neutralization in an electron beam device or system.

Another object is to provide improved methods of and means for controlling space charge neutralization in an electron beam device with negligible accompanying electron velocity variation.

Another object of the invention is to control the density of an electron beam in accordance with the frequency characteristics of two electric currents and thereby accomplish the mixing of the currents to produce a current of an intermediate frequency.

Other objects will be apparent from the description of the invention as hereinafter set forth in detail and from the drawings made a part thereof in which Figure 1 is adiagrammatic sketch of arrangement of the basic elements of the invention; Figure 2 is a diagrammatic sketch 1 of the elements of Figure 1 appliedto a reflexklystron; Figure 3 is a diagrammatic sketch of another embodiment of the invention applied to a modulator structure; Figure 4 is a diagrammatic sketch showing the application of modulation to the arrangement disclosed in Figure 1; Figure 5 is a plot showing the relation of (1) values of the ratio of collected current to beam current to (2) the values of the beam current in the circuit set forth in Figure 3; Figures 6, '7 and 8' are associated characteristic curves of the arrangement shown in Figure 3; Figure 9 is a diagrammatic sketch of a further embodiment of the invention applied to a signal mixer; Figures 10 and 11. are associated characteristic curves of the arrangement shown in Figure 9; and Figures 12a, 12b, 12c,

12d and l2e are associated characteristic curves of the arrangement shown in Figure 9.

. Similar reference characters are applied to similar elements throughout the drawings.

Referring to Figure 1, numeral i represents a. spherical cathode, as a source of a focussed electron beam. Additional focussing may be obtained by a focussing ring or electrode 2 positioned near the periphery of cathode I and maintained at an appropriate negative potential, in any conventional manner.

The electrons in the beam are projected towards and accelerated by electrodes 3, 4 and 5, which are maintained at positive potentials V3, V4 and V5 respectively, by electric source 6. In the center of electrode 5, and in the axis of the beam, is aperture 1. Aligned with aperture 1 is a through aperture 1. The device may be grounded as at 9.

In operation, electronsin the beam are accelerated by the electrodes 3, 4 and 5 toward the aperture I and after passing therethrough are collected by Faraday cage 8.

The amount of current collected under the influence of the various potentials placed upon electrodes 3, 4 and 5 is a measure of the fineness or the density of the beam or its dispersion.

The spherical cathode I and focussing electrode '2 should'produce abeam converging at aperture 1 into a fine focus. However, a space charge is built up withinthe beam due tothe like charges I of the electrons constituting the beam and the beam electrons collide with the residual molecules of the medium causing ionization of molecules of the medium and thereby creating additional electrons. The electrons created by the ionization of the medium are of relative low velocity,'are

repelled by the electrons in the'beaxrnand drift away from the beam. The ions, being charged positively, are attracted to and trapped within the beam where they neutralize the space charge created by the charges of electrons in the beam itself. 5

Without the electrode 4, the field outside the substantially field-free space between electrodes 3 and 5 would extend through the aperture or window in electrode 3, causing ions in the field- 'free space to drift towards the cathode I. The

presence of'electrode 4 with its positive poten tial effectively closes the aperture in electrode 3 insofar as field penetration from without and ion drift from within the free-field space is 'concerned. A grid structure across the aperture in electrode 3 would operate similarly, but would have the disadvantagethat considerable grid cur rent would result, whereas electrode 4 drawsnegligible current; There is thus provided a field-free region through which the beam passes without the dispersion that would otherwise occur and with negligible change in the velocities of the electrons. It is apparent, then, that with variations of the forces establishing the field free space, the configuration-of the said space may be varied and thus the density of thebeam may be varied. The beam may thus be concentrated to its optimum fineness or dispersed and thereby control may be had of the proportionof the electrons of the beam that pass through aperture 1. Modulation or amplification is thus provided.-

The embodiment of the invention in various structures to obtain the clesired'results will be disclosed hereinafter, including the embodiment of the invention to provide the mixing ofcurrents of differentfrequencies-to obtain a current of an intermediate frequency. a

In practice, it is preferable in'general that the" potential'placed on electrode 3 should be higher than that on electrode 5' and thepotentialplaced on electrode 4 should be less than that on elec- The magnitudes of these potentials, however, depend on the geometry of the device.

trode 5.

In the arrangement tested, the potentials on electrodes 3, 5 and 4 were: V3=300'volts, V5=290 volts and V4'=280'volts. .1

. Referringto Figure 2, the inventionis com bined with, a refleX-klystron structure. Here. the electrode .5 of Figure 1 has added to it the side wall 5a to form the resonantcavity. Elecacregarr trodes 3a and 4a are added to create a field-freedrift space between wall of the resonant cavity" and repeller electrode II], which is connected to a source of negative potential ('Vr) and is substitutedfor the Faraday cage 8. Electrode 4 controls the field penetration at the aperture in electrode 3 and therefore controlsthe configuration of the field-free region, as previously 'described.

The modified reflex-klystron operates in the conventional manner, as is well known in the art, but with greater eficiency because the electron beam is not dispersed by theotherwise present space charge, especially in the. drift space where debunching is prevented.

Referring to Figure 3, the invention is incorporated into a modulator structure. Here, the electron beam is supplied by cathode I and the electrons thereof are accelerated by electrode 3 which is in the form of a grid and is positioned. in the axis of the beam. The electrons are also accelerated by electrodes 4 and 5. The electrons pass through aperture 1 in electrode 5 and are collected by the electrode l0. Load resistance II and electric source l2 are connected in series with each other between electrode Ill and ground 9. Modulating electrode 4 is connected to one terminal the secondary of transformer l3, the primary of which is connected to the source l4 of the modulating. current. The other terminal of the secondary of transformer l 3 is connected through source 6 to ground 9. Electrodes 3 and 5 are connected to source 6. Electrode 4 is biased as to electrodes 3 and 5 by connecting electrodes 3 and 5 to the end positive pole of source 6 and electrode 4 is connected to an intermediate positive pole of source 6. The potentials of electrodes 3 and 5 above ground is designated as Va and the bias potential of electrode 4 below electrodes 3 and 5 is designated as Vb. The electric potentials induced in the secondary of transformer 13 are designated as es sin wt.

In operation, if the potential on electrode 4 be held constant, biased as to electrodes 3 and 5 by Vb, the beam would be maintained at its optimum density. However, when the potential on electrode 4 is varied by the potentials applied from source I4 through transformer l3, the beam is concentrated and dispersed according to the characteristics of source I4. This results in a modulated current being collected by electrode Hit It is apparent thatthe same effect on the beam could be had by varyingthe potential. on grid 3 with respect to the potential onelectrode 5... This may be accomplished by connecting the' secondary of transformer l3 between grid 3 and source 6. It is also apparent that with this latter arrangement, a comparatively weak signal impressed'upon electrode 3 may control the density ofv the beam and provide an amplification of. the signal through the load resistance II.

The arrangements shown in Figure 4 "are the same as those in Figure l exceptthat electrode 4 has impressed upon it the modulating voltage es sin wt through transformer l3 and a load circuit, including load resistance H, electric source I2 and ground 9, is substituted for Faraday cage &- and associated indicating instruments, not shown, as heretofore described.

H The construction and circuit arrangements of Figure 4 are similar to those in Figure 3 except that the aperture in electrode 3 in Figure 4is open whereas electrode 3 in Figure 3' is in. the form of a grid. There is thus provided in Figure 4 a new type of amplifier which depends its-operationon controlling the density-of a comby a restricted aperture in the electrode at theexit point of the region.

The operating characteristic curves are shown in Figures 5, 6, '7 and 8. In Figure 5 is plotted the relation between (1) the ratio of current flowing to collector I0 (Io) to the total beam current (Ib) to (2) the total beam current (Ib). The solid line in Figure 5shows this relation when V4=V3=V5 and the dotted line in Figure 5 shows this relation where V4 is less than V3 and V5.

It will be noted from these curves that the ratio of Io to It decreases for increases of It. This isapparently due to a decrease in effective trapping.

When the potential on electrode 4 is reduced with respect to electrodes 3 and 5, the region through Which the beam passes is less field-free and the positive ions, created by ionization of the The coincidence of'the solid curve with the" dotted curve at low values of Ib, when the beam dispersion due to space charge is negligible, indicates that the focussing of the electron optical system is not disturbed by the variations of potentials placed on electrode 4.

In Figure 6' is plotted the relation between the current collected by electrode I3 (10) to differences in potentials between electrode 4 and electrodes 3 and 5 (Va) for the two beam currents I1 and I2, Figure 5. The value of In for I1 (dotted curve) in Figure 6 is practically constant over the complete range of Va, which indicates that the defocussing of the beam is negligible, as there are no space charge limitations. The solid curve (I2) indicates the actual change in current density due to the variations in the potentials placed on electrode 4 relative to the potentials placed on electrodes 3 and 5.

The effect of applying a signal voltage (es sin wt) on electrode 4 is shown graphically by Figures 6, '7 and 8, the ordinate axis of Figure 6 being aligned with the ordinate axis of Figure 7 and the abscissa axis of Figure 6 being aligned with the abscissa axis of Figure 8. Values of the varying voltage (es sin wt), biased by the value Vb (Figure 7), are projected by projection lines 2| onto Figure 6 and the corresponding value of Is are projected onto Figure 8 (Io versus time) to obtain the modulated values, 1b.

Referring to Figure 9, the invention is shown incorporated into a mixer or an arrangement for converting currents of two frequencies to a different frequency, called the intermediate fre- .quency. Here, a focussed electron beam is produced by spherical cathode I, with supplementary focussing electrodes 2, as previously disclosed. The beam passes through control grid 91 and accelerating grid g2, through the cavity form-ed by electrode 5, and through the aperture 1 of electrode 5. The electrons are collected by electrode Ill. The load circuit consists of primary of transformer l5, electric source l2 and ground 9. A capacitor IE is connected in parallel with source 12 to by-pass the radio frequency currents. The output of the device is the secondary of transformer l5.

Control grid 471 is biased above ground or neutral potential. by electric source I! and has im- I 8 of the first frequency current. Electrode has from source 6, varying potentials from the source tialfrom source 20, which biasses grid g2 with re- 2! is connected in parallel with source 6 to by-pass other.

The operation of the device is explained graphically by the characteristic curves in Figures 10, 11, 12a, 12b, 12c, 12d, and 12e. The effect of potential changes on grid g1 alone is observed by maintaining constant the potential on grid g2. There being no change in potential of source 6,

constant.

A varying potential 61, that of the first frequency current source I3, is placed upon grid g1, with a biasing potential from source H. The value of the current in the beam passing between the grid 92 and the electrode 5, Ik, is determined by the usual relation Aik=Gm1Ae 1, where Gmi is the transconductance of the device. The plot of this relation is shown in Figure 11. The relations between the values of the current Ik and the current collected by electrode IE], Ic, is observed and is plotted in Figure 10. It is thus possible to obtain the values of current Ic for variations of egl by projecting (lines 2|) corresponding values of I11 determined from the curve in Figure 11 to equal values of Ik in Figure 10, to obtain the corresponding values of Io. The curve to, FigurelO, indicates the instantaneous or time (t) values of Io.

constant, the value of Ik is constant and the efiect of placing a varying potential (e2 sin wt) on grid 92 can be determined as disclosed with reference to Figures 6, '7 and 8, for individual values of Ik- A series of curves can, therefore, be plotted, as in Figure 12a, for the constant values of 1k, of C1, C2 and C3, respectively.

A composite group of curves may then be plotted, as in Figures 12a, 12b, 12c, 12d and 12e, in

(Figure 12d) may be projected by lines 2| onto Figure 120 to obtain instantaneous values of Ik. The biased values of 6g2 (Figure 12b) may be projected by lines 2| onto Figure 12a. By selecting the individual curve of Figure 12a, corresponding to the instantaneous value of 11; obtained from Figure 120, the instantaneous values of ted as in Figure 12c.

It will be apparent that the control grid 9 the constructions and circuits shown in Figures 2, 3 and 4 by placing grid g1 in efiective control position in relation to cathode I in the said figures as is shown in Figure 9. The arrangements thus combined provide other mixing circuits.

There is thus disclosed a'method of and apparatus for establishing a field-free region in micro-- wave devices and neutralizing the space charge of the region by trapping the positive ions within the region to concentrate or sweep out the ions to accomplish current modulation amplification or mixing. I

I claim as my invention: l. A reflex-klystron device including a source pressed upon it varying potentials from the source 3 impressedupon it a positive potential from electric source 6. The potentials impressed upon accelerating grid g2 includes a positive potential I9 of the second frequency current and a potene spect to electrode 5 and collector 10. A capacitor the radio frequency currents. The potentials of sources l2 and '6 are substantially equal to each the bias of grid g2 with relation to electrode 5 is When the potential on grid i is maintained which the biased instantaneous values of egl the intermediate frequency current may be plot-..

and its associated circuit may be combined with,

currents;

of; electrons, means for providing a focussed beam of electrons therefrom, a-resonant cavity,arefiect-.

ing electrode, and a plurality of accelerating electrodes that define boundaries of a. substantially field-free region in said medium surrounding said cavityeach of said accelerating electrodes having an" aperture therein the centers of which aper-' tures are in the axis'of said beam, and means for in the vicinities of said apertures are controlled. '2. A refiex-klystron device including a source of electrons, means for providing a focussed beam of electrons therefrom, a resonant cavity, a reflecting electrode, and a plurality of accelerating electrodes that define boundaries of a substantially ficld-free region in said medium surrounding saidcavity each of said accelerating electrodes having an aperture therein the centers of which apertures are in the axis of said beam, and means for variably controlling the density of said beam comprising means for imposing varying electric potentials upon said region in the vicinities of the said apertures, whereby the configuration of said field-free region in the vicinities of said apertures are variably controlled.

'3, Apparatus for mixing two currents of difierent frequency characteristics to obtain a current of an intermediate frequency comprising: meansfor providing a focussed electron beam in a gaseous medium having a field-free region, means for varying the density of said beam in one region of said medium in accordance with the first of said currents, means for projecting said beam through a second and substantially field-free ref gion of said medium whereby the space charge within'the beam is'neutralized, a restricted aperturedevice, means for projecting the said beam' through said restricted aperture; means for collecting the electrons of the beam that pass through the said aperture, and'means for varying the configuration of the said second region in accordance with the second of said currents, whereby the said beam is concentrated and dispersed as the said beam approaches said aperrent of an intermediate frequency comprising:'

means for providing a focussed electron beam in a medium having a field-free region, means for providing an electric field in a first region in the path of said beam, means for varying the:

said electric field in accordance with the characteristics of the first of said currents, means for projecting the said beam through a second and substantially field-free region of said said medium whereby the space "charge within the beam is neutralized, a restricted aperture device, means for passing the said beam through said restricted aperture, means for collecting the electrons of the beam that pass through the said aperture, and means for varying the configurations of the said second'region in accordance with the characteristics of the second of said currents whereby the said beam is concentrated and dispersed as the said beam approaches said aperture and the portion of the electrons. in said beam passing through said aperture is varied in'accordance with the characteristics of the second of said ,rent of an intermediate frequency comprising:

means for providing ajfocussed electron beam in [amedium having a field-free region, means for; Ivarying the density of said beam in one region of .said medium in accordance with the frequency characteristics of the first of said currents, means .for projecting the said beam through a second and substantially'field-free region of said me-,

dium, the entering'boundary of said second region being a grid and the exit boundary thereof be- .ing a restricted aperture, means for impressing position potentials on said boundaries, the grid boundary being biassed positive with respect to said aperture boundary, means for superimposing on said grid a potential varying in accordance with the frequency characteristics of the second of said currents, whereby the said beam is concentrated and dispersed as it approaches said aperture in accordance with the frequency characteristics of the second of said currents, and means for collecting the electrons that pass through said aperture as the current of said intermediate frequency.

6. An electron device including: an electron beam source and a focussed electron beam emitted thereby, a series of electrodes consisting of three apertured electrodes and a solid electrode juxtaposed said beam. the centers of the apertures in said apertured electrodes and the center of said solid electrode being positioned in the axis of the beam, whereby an uninterrupted path for the electrons of said beam is provided from said source to said solid electrode, means for impressing positive electric potentials on said apertured electrodes. whereby a substantially field-free region is established between said apertured electrodes,

and means connected to said solid electrode for collecting the electrons in said beam that pass through said apertured electrode.

7. An electron device including: an electron beam source and a focussed e ectron beam emitted thereby, a series of electrodes consisting of three apertured electrodes and a solid electrode juxtaposed said beam, the centers of the apertures in said apertured electrodes and the center of the said solid electrode being positioned in the axis of the beam, whereby an uninterrupted path for the electrons of said beam is provided from said source to said solid electrode, means for impressing positive electric potentials on said apertured electrodes, whereby a substantially fieldfree region is established between said apertured electrodes, the potentials on the electrodes near- 7 est the said source and the said solid electrode, respectively, being positive and substantially equal to each other, the potential on the intermediate apertured electrode being positive and Of a less potential than the potentials of the other apertured electrodes, and means connected to said solid electrode for collecting the electrons in said beam that pass through said apertured electrode.

8. An electron device including: an electron beam source and a focussed electron beam emitted thereby, a series of electrodes consisting of four apertured electrodes and a solid electrode juxtaposed said beam, the axis of the apertures in said apertured electrodes and the axis of the said solid electrode being positioned in the axis of the beam, whereby an uninterrupted path for the electrons in said beam is provided from said source to said solid electrode, means for impressing positive electric potentials on said apertured electrodes whereby a substantially field-free region is established between said apertured electrodes the potentials on the electrode nearest the said source and the electrode nearest said solid electrode, respectively, being substantially equal to each other, the potentia1 on the two intermediate apertured electrodes being positive and of a less potential than those of the other apertured electrodes, a resonant cavity positioned between the two said intermediate electrodes and means connected to said solid electrode for reflecting the electrons in said beam that pass through said apertured electrodes in a reverse direction into said cavity, whereby electric oscillations are produced in said cavity.

9. An electron device including: an electron focussed beam source and an electron beam emitted thereby, a series of electrodes consisting of three apertured electrodes and a solid electrode juxtaposed said beam, the centers of the apertures in said apertured electrodes and the center of the said solid electrode being positioned in the axis of the beam whereby an uninterrupted path for the electrons of said beam is provided from said source to said solid electrode, means for impressing positive electric potentials on said apertured electrodes whereby electric fields are established between said apertured electrodes, means for supplying a modulating current, means for impressing potentials on said intermediate apertured electrode in accordance with the frequency characteristics of the said modulating current, and means connected to said solid electrode for collecting the electrons in said beam that pass through said apertured electrodes, whereby the said collected electrons constitute a current the frequency characteristics of which correspond to those of the said modulating current.

10. An electron device including: an electron beam source and a focussed electron beam emitted thereby, a series of electrodes constituted of three apertured electrodes and a solid electrode juxtaposed said beam, the centers of the apertures in the said apertured electrodes and the center of the solid electrode being positioned in the axis of said beam and the apertures of the two apertured electrodes nearest the said source being in the form of grids, means for supplying a current of a first frequency, means for supplying a current of a second frequency, means for impressing on the said apertured electrode closest to said source a constant positive potential and superimposed thereon potentials corresponding to said first frequency current, means for impressing on the said apertured electrode next closest to said source a constant positive potential and superimposed thereon potentials corresponding to said second frequency current, means connected to said solid electrode for collecting the electrons of said beam reaching said solid electrode whereby the collected electrons constitute a current of said intermediate frequency, and means for utilizing said collected current.

11. A gaseous medium electron device including a source of electrons, a plurality of accelerating electrodes which define boundaries of a substantially field-free region in said medium, said electrodes having coaxial apertures therein, means for focusing electrons from said source into a beam coaxial with said apertures, means for accelerating said beam to pass through said electrode apertures, means for applying potentials to said boundaries of said region to establish said field-free region thereby creating ions in said region by collisions of said electrons with the molecules of said medium and hencetrapping saidions within said beam within said region to neutralize the space charge in said beam in said region, and means for imposing an electriepotential upon the boundaries of said region in the vicinity of the said aperture closest to said source I for controlling the density of said electron beam passing through said region. 7 I JOHN H. COLEMAN.

REFERENCES CITED The following references are of record in the file of this patent:

Number V Adler 'Ju ne,l3, 1950 

